6MBPS BUT POSSIBLY AS HIGH AS 10MBPS) THAN NETWORKS THAT USE DSSS
1.6Mbps but possibly as high as 10Mbps) than networks that use DSSS.
Direct-Sequence Spread-Spectrum
DSSS works somewhat differently from FHSS.With DSSS, the data is divided and simulta-neously transmitted on as many frequencies as possible within a particular frequency band(the channel). DSSS adds redundant bits of data known as chipsto the data to representbinary 0s or 1s.The ratio of chips to data is known as the spreading ratio. As the ratioincreases, the signal becomes more immune to interference, because if part of the transmis-sion is corrupted, the data can still be recovered from the remaining part of the chippingcode.This method provides greater rates of transmission than FHSS, which uses a limitednumber of frequencies but fewer channels in a given frequency range. In addition, it alsoprotects against data loss through the redundant, simultaneous transmission of data.However, because DSSS floods the channel it is using, it is also more vulnerable tointerference from EM devices operating in the same range. In the 2.4GHz to 2.4835GHzfrequency range employed by 802.11b, DSSS transmissions can be broadcast in any one of14 22MHz-wide channels.The number of center-channel frequencies used by 802.11DSSS devices depends on the country. For example, North America allows 11 channelsoperating in the 2.4GHz to 2.4835GHz range, Europe allows 13, and Japan allows 1.Because each channel is 22MHz wide, channels may overlap each other.With the 11 avail-able channels available in North America, only a maximum of three channels (1, 6, and 11)may be used concurrently without the use of overlapping frequencies.How Wireless Works
The 802.11 standard provides two modes for wireless clients to communicate: Ad Hoc andInfrastructure.The Ad Hoc mode is geared toward an unstructured network of wirelessclients within communication range of each other. Ad Hoc networks are created sponta-neously between the network participants, resulting in a fully meshed network. InInfrastructure mode, APs provide for a more permanent structure for the network. An infras-tructureconsists of one or more APs as well as a distribution system (such as a wired net-work) behind the APs, which tie the wireless network to the wired network. Figures 9.1and 9.2 demonstrate the two modes, Ad Hoc and Infrastructure.Figure 9.1Ad Hoc Network ConfigurationLaptop LaptopPDA WorkstationIn an Ad-Hoc network, each participant is free tomake a connection with any one otherparticipant directly.Figure 9.2Infrastructure Network ConfigurationIn Infrastructure mode, wireless clients only communicate directlywith the Access Point.LaptopTo distinguish different wireless networks from one another, the 802.11 standard definesthe Service Set Identifier (SSID).The SSID can be considered the identity element that“glues” together various components of a wireless LAN.Traffic from wireless clients that useone SSID can be distinguished from other wireless traffic using a different SSID. Once thecorrect network mode has been configured on the wireless client, an AP can determine whichtraffic is meant for it and which is meant for other wireless networks by using the SSID.The 802.11 traffic can be subdivided into three parts:■
Control frames Control frames include such information as request to send(RTS), clear to send (CTS), and acknowledgement (ACK) messages.■
Management frames Management frames include beacon frames, proberequest/response, authentication frames, and association frames.■
Data frames Data frames are, as the name implies, 802.11 frames that carry data.That data is typically considered network traffic, such as IP encapsulated frames.These three types of frames can be mapped to different layers within the Open SystemsInterconnect (OSI) networking model, which are described in the next section.